With the arrival of new technology there is always a certain amount of confusion, especially so it seems with Digital and High Definition Television. This technology is quite complex, and could take up much more space than we have here, but we will try to hit most of the high points.
With the TVs we have at home now, an analog broadcast signal carries intensity and color information to each scan line of the picture tube. Our TV signal has 525 scan lines for the image, and each image is refreshed every 1/30th of a second. Half of the scan lines, meaning the picture, are painted every 1/60th of a second in what is called an interlaced display. The horizontal has something like 500 lines of resolution for a color set. This was amazing 50 years ago, but not today. The lowest resolution computer monitor today has 640 by 480 pixels, and most people use 800 by 600 or 1024 by 768. This is also progressively scanned, offering higher clarity.
Interlaced or progressive refers to the way the image is “painted” on the screen. In an interlaced format, the screen shows every odd line at one scan of the screen, and then follows that up with the even lines in a second scan. This is the way analog TV works now. Since there are 30 frames shown per second, the screen shows one half of the frame every 1/60 of a second. As screens get large the problem with interlacing is flicker. For smaller screens this is less noticeable. Progressive scanning shows the whole picture, every line in one pass, every 1/60 of a second. This provides for a much smoother picture, especially on larger sets, but uses more broadcast bandwidth.
New satellite systems and DVD’s, use digital encoding, offering a very clear picture. Right now these systems convert the digital information to analog, so that you can see it on your analog TV. The image looks great compared when compared with regular TV, but it could look twice as good if the analog conversion didn’t happen. There is now a trend to change our TV sets to digital, with digital signals driving your TV directly, and a much better picture. New digital TV sets accept pure digital signals, and provide a high-resolution picture that is extremely crisp and lifelike.
Most talk regarding Digital Television (DTV) is about the transmission of the digital television signal, and their reception and display on a digital TV set. Digital signals might be broadcast over on air, or transmitted via cable or satellite. In any event, a decoder receives the signal and, in digital form, directly drives the digital TV set.
Within the world of digital broadcast is a type of DTV called High Definition Television, or HDTV. HDTV is high-resolution Digital Television combined with Dolby Digital surround sound (or AC-3). HDTV is the highest DTV resolution in the new set of formats. This combination creates a stunning image with stunning sound. HDTV requires new production and transmission equipment at the broadcasting TV station, as well as the new equipment for reception at the viewer’s end. A higher resolution picture is the main point for HDTV. Imagine 720 or 1080 lines of resolution compared to the 525 lines you are used to here in North America (or the 625 lines in Europe) — it’s a huge difference!
Of the eighteen DTV formats, only six are HDTV – three of which are based on progressive scanning and three on interlaced scanning. Of the remaining formats, eight are Standard Definition TV (SDTV has four wide-screen formats with 16:9 aspect ratios, and four conventional formats with 4:3 aspect ratios). The remaining four are Video Graphics Array (VGA) formats. Broadcast stations are free to choose from among these formats.
The HDTV formats:
- 480i – 640×480 pixels interlaced,
- 480p – 640×480 pixels progressive
- 720p – 1280×720 pixels progressive
- 1080i – 1920×1080 pixels interlaced
You will not need to know which format your favorite TV station has chosen. The decoder will do that for you automatically, supplying the best resolution output to your digital TV based on its capabilities. So, although the Digital Television and High Definition Television world may seem a bit daunting, the ability to lay on the couch pushing a single button to move you through the world of entertainment, that is broadcast television, will still be there. Don’t you love technology?